1. Field of the Invention
The present invention relates to improved well drilling fluids and methods which prevent damage to water sensitive formations while drilling.
2. Description of the Prior Art
While drilling oil and gas wells into and through water sensitive subterranean formations, i.e., formations containing water swellable clays and other fines capable of migrating when disturbed, the contact of the clays and fines with an aqueous drilling fluid can cause damage to the formations. That is, the clays and fines are normally stable and do not present an obstruction to the flow of hydrocarbons through the subterranean formations containing them. However, when contacted with aqueous fluids which are foreign to the formations, the clays can swell and/or the fines can migrate in the formations causing losses in the permeabilities of the formations. Also, the swelling of clays can cause strength reductions in a subterranean formation which can result in the collapse of shale into the well bore.
Heretofore, it has been a common practice to include salts in aqueous drilling fluids to prevent or minimize damage resulting from the swelling of clays and migration of fines. The salts, such as potassium chloride, calcium chloride, ammonium chloride and the like, are typically dissolved in the aqueous fluid utilized in the drilling fluid. The salts effect an ion exchange with the charged particles comprising the clays and fines, and are often very effective in preventing swelling and/or migration.
In many areas where oil and gas wells are presently being drilled, the disposal of fluids having high salt or halide contents has been restricted to protect the environment. Such restrictions require that drilling fluids having high salt concentrations be disposed of in a special manner which is often expensive and difficult.
Thus, there is a need for improved aqueous drilling fluids and methods which prevent or minimize damage to water sensitive formations whereby salts, and particularly chloride containing salts, are minimized or not included in the drilling fluids.